Feed-forward error control was originated by Harold S. Black in 1924, a precursor by several years of his more famous accomplishment of feedback control. Through 1960, it was viewed somewhat as a curiosity, and was used only in the context of a McMillan circuit, which fused the two Black concepts of feedback and feed-forward. Other feed-forward circuits have been known. See, for example, H. Seidel, "A Microwave Feed-Forward Experiment," Bell System Technical Journal, November, 1971, pages 2879-2899.
Such a prior art arrangement is shown in FIG. 1. There is shown a feed-forward distortion cancellation circuit. As is shown, an input 1 (x) is applied to an amplifier 2, producing a non-linear output 8 equal to Ax+A.sub.3 x.sup.3. The non-linear quantity A.sub.3 x.sup.3 is illustrative of the distortions due to the non-linearities associated with the amplifier 2. The output 8 is then input to an attenuator 4, and a first input of a summing device 6. The feed-forward path 11 provides the quantity -A.sub.3 x.sup.3 to a second input of the summing device 6, thus producing the linear quantity Ax to the output 7.
A second such prior art arrangement is shown in FIG. 2. There is shown a feedforward distortion cancellation circuit with active control of a first cancellation loop. This circuit is similar to the circuit of FIG. 1 except that a variable attenuator and phase shifter 22(B1) is arranged with a comparison circuit 23 and amplitude and phase control line 35 to maintain the relationship B1*B2=A.
A third such prior art arrangement is shown in FIG. 3. There is shown a feedforward distortion cancellation circuit with active control of both cancellation loops. This circuit is similar to the circuit of FIG. 2 except that a second variable attenuator and phase shifter 49 (B3) is arranged with a second comparison circuit 50 and a second amplitude and phase control line 56 to maintain the relationship B3*B4=B2.
A fourth such prior art arrangement is shown in FIG. 4. There is shown a feedforward distortion cancellation circuit with active control of both cancellation loops and an identification or pilot signal, also known in the art as a "TAG," used as a reference. Note the pilot signal 80 is applied to the output of amplifier 62 and as one input to the second comparison circuit 70. See, for example, FIG. 7 in the above-cited BSTJ article by H. Seidel.